1. Field of the Invention
This invention relates generally to the use of chemical vapor infiltration (CVI) to bond diamond particles together resulting in a composite article comprising diamond particles in a diamond deposit matrix. This invention relates more specifically to the use of a forced flow chemical vapor infiltration approach in which carbon vapor reactant gases are forced to flow through a preform of densely packed, using particle packing theory, diamond particles and/or fibers resulting in a densification of the preform so as to form a strong composite diamond article.
2. Prior Art
The plasma assisted chemical vapor deposition (CVD) of diamond films for coatings using low pressure, moderate temperature processing conditions has been studied actively for the past ten years. However, current deposition techniques are often not practical for the fabrication of thick (few millimeters) articles because the deposition rate is low (typically 1 .mu.m per hour). Various approaches are under development for increasing the deposition rate but none have been entirely satisfactory to date. Often attempts to increase the rate of deposition have resulted in inferior quality deposits. For example, running the reactor with a high reagent gas flow rate and/or concentration tends to result in the deposition of carbon films instead of diamond.
Although these solutions have had some success in the deposition of chemical vapors for coating purposes, they have not been successful in achieving fast deposition and creation of large size samples or articles. The lack of any satisfactory process for creating large size samples or articles of diamond material within a reasonable length of time, that is, having a deposition rate fast enough to be practical, and the inability to tailor the final properties of the material currently are the bane of the CVD diamond art.
The rapid process for the preparation of diamond articles disclosed herein overcomes the above problems by being able to rapidly fabricate large size diamond components using a unique CVD process which involves chemical vapor infiltration (CVI) of diamond particles using a carbon vapor.
In the CVI process, a matrix is chemically vapor deposited within a porous preform to produce a composite material. CVD in general results in the production of a coating, while CVI in specific results in the production of a composite article. Thus, CVI is considered a specialized form of CVD. The preform may consist of particulates, fibers, or any other suitable constituents or materials which will form a porous medium. The preform to be subjected to CVI is placed in a modified CVD reactor. Gaseous CVD reagents penetrate the pores of the preform and deposit onto the surfaces of the particles. As the deposition process continues, the particles are coated and grow, and consequently the spaces between the particles become smaller. Eventually, the particle coatings interlock and the particles are bonded together by the coating. This coating is the matrix which along with the original particles constitutes the composite.
CVI processes can be classified into five types as illustrated in FIG. 1. The bases for the classification are (1) whether the preform is uniformly heated, that is isothermal, or contains a thermal gradient and (2) the manner in which the gaseous reagents contact or flow through the preform. CVI Type I (FIG. 1a) is an isothermal process in which the reagent gases surround a preform maintained at a uniform temperature and enter the preform via diffusion. CVI Type II (FIG. 1b) is a thermal gradient process in which the reagent gases flow across and contact the cold side of a preform having a thermal gradient across it and enter the preform via diffusion. CVI Type III (FIG. 1c) is an isothermal forced-flow process in which the reagent gases flow through the preform via a forced flow. CVI Type IV (FIG. 1d) is a thermal gradient process in which the reagent gases are forced to flow through a preform having a thermal gradient across it from the cooler surface to the warmer surface. CVI Type V (FIG. 1e) is an isothermal pulsed flow process in which the reagent gases flow into and out of the preform via a pulse flow, creating a cyclical evacuation and back filling of the preform. A sixth type of CVI process, CVI Type VI, not shown in FIG. 1, is a pulsed flow process similar to that of CVI Type V, but in which a thermal gradient is created across the preform.
In the Type I isothermal process, the preform is maintained at a uniform temperature, and the reagent gases flow through the furnace. There is no provision for forcing the reagent gases to flow through the preform, and therefore the reagent gases enter the preform only by chemical diffusion. Since chemical diffusion is slow, the Type I isothermal infiltration method typically requires very long infiltration times, generally on the order of weeks, and there is often a density gradient from the surface toward the center of the preform. The Type II processes, while achieving somewhat higher diffusion rates, also requires very long infiltration times, and is thus impracticable.
For the forced-flow (Type III and Type IV) CVI processes, the reagent gas stream is constrained to flow through the preform. The Type IV thermal gradient-forced flow infiltration method has the advantage of significantly reduced infiltration times and, by proper adjustment of process variables, achieves a relatively unitary density throughout the component. Compared to the Type III isothermal forced-flow technique, processing time for the Type IV thermal gradient-forced flow infiltration method typically is reduced by a factor of about 50.
The greatest use of CVI processing has been to fabricate carbon matrix-carbon fiber and silicon carbide matrix-carbon or silicon carbide fiber composites for aircraft brakes, rocket nozzles, nose cones and other aerospace components. Fiber preforms with initial densities of 40-50% have been densified in large scale commercial processing to 85% of theoretical density. See the International Encyclopedia of Composites, Ceramic Matrix Composites CVI: Chemical Vapor Infiltration (VCH, New York; Stuart M. Lee, Editor; W. J. Lackey, contributing author), which article is incorporated herein by this reference, and the references cited therein, and made a part hereof. Particulate preforms consisting of nuclear fuel particles also have been consolidated successfully using CVI processing. Consolidated densities of 85-90% are possible since a ceramic body remains permeable until a density of approximately 90% theoretical is reached.
A process and apparatus for the preparation of fiber reinforced composites by chemical vapor deposition has been patented by one of the present inventors under U.S. Pat. No. 4,580,524. The '524 patent discloses a process and apparatus for creating a thermal gradient across a fibrous preform, thus creating a thermal gradient process as described above, and directing a flow of gaseous material into the preform, thus completing the forced flow thermal gradient process similar to the CVI Type IV process described above. In turn, the gaseous material is deposited into the preform so as to produce a fiber reinforced ceramic material. The apparatus comprises a means for supporting the preform, means for heating and cooling different surfaces of the preform, and a means subjecting the preform to a forced flow. An apparatus somewhat similar to this type is suitable for the rapid process for producing diamond articles disclosed herein. However, this apparatus has never been utilized to carry out a rapid process for the preparation of diamond articles as it was not thought theoretically possible to be able to vaporize a carbon source and deposit diamond by a chemical vapor infiltration process applied to packed diamond particles due to the unique activation process required.
Diamond deposition occurs via a plasma assisted process. The presence of atomic hydrogen, H.multidot., is accepted as necessary to grow diamond films. To produce atomic hydrogen, some plasma activation is necessary. Known methods for achieving plasma activation include microwaves, RF power, tungsten filaments, acetylene torches, and DC arcs. The '524 patent is a conventional CVD/CVI furnace, and has, at best, only a limited capability to activate a plasma process.
A method for preparation of diamond ceramics is disclosed in U.S. Pat. No. 4,882,138 to J. M. Pinneo. The '138 patent discloses a typical method for consolidating diamond particles in which a compacted aggregation of diamond particles is contacted with a gaseous carbon source and atomic hydrogen in a plasma enhanced reactor. The diamond particles are previously compacted through mechanical pressure, vibratory or other means, and are subjected to plasma enhanced chemical vapor deposition at pressure from about 10.sup.-2 torr to 10.sup.3 torr at temperatures ranging from about 800.degree. C. to about 1450.degree. C. The '138 patent discloses a process which relies on the typical chemical vapor deposition process and does not disclose the use of a forced flow CVI process. Therefore, the '138 patent has the disadvantage of not being a rapid process, due to its lack of a forced flow, and is more similar to Type I and Type II CVI processes discussed above.
Developing a forced flow CVI process for the deposition of diamond is not obvious in light of the '138 patent. The method, parameters, and the appropriate fixturing disclosed herein which allows the forced flow CVI in a diamond particulate preform are not obvious as evidenced by the near universal belief of those skilled in the art that diamond deposition is a surface process. Current art has not fully extended to the forced flow deposition of diamond within a preform to obtain composites.
The prior art technology has several disadvantages when applied to the preparation of diamond articles. Although a thick film deposition can be achieved with the current technology, a 1 mm substrate would take over one month to be deposited at the typical deposition rate of about 1 micron per hour. Obviously, using the prior art techniques, the length of time to deposit enough diamond to fabricate a large article would be prohibitively long. Further, the chemical vapor infiltration of diamond has been demonstrated using only relatively slow chemical diffusion to densify the composite. Current chemical diffusion technology also is very time intensive, taking over one month to partially densify a 3 mm thick disc.
The prior art technology has failed to fully recognize both the use of a plasma in a forced flow CVI process and the ability of a plasma to infiltrate a preform using a forced flow CVI process to deposit diamond within the interior of the preform and not merely on the surface of the preform. It generally has been the experience that forced flow processes are detrimental to the existence of plasmas, as the forced flow theoretically upsets the plasma. Further, plasma enhanced chemical vapor deposition has heretofore been considered a surface deposition technology, resulting in the deposition of the selected material only on the surface of the substrate. Although the use of a plasma to deposit diamond within an aggregation of diamond particles has been described in the '138 patent, the use of a forced flow in connection with a plasma to achieve CVI in a densely packed (using particle packing theory) diamond preform has not been disclosed or theorized in the prior art.
Typically, the substrates subjected to CVD have been solid wafers or particles contained in crucibles. Both of these forms have disadvantages when used in a forced flow CVI process. It is difficult if not impossible to force flow reagent gases into or through a solid wafer. Therefore, a surface coating typically results on a solid wafer subjected to CVI. A forced flow obviously will disrupt a powder substrate. Consequently, forced flow generally is not used in the CVI of powder substrates. The CVI reactor developed for the present invention also includes a novel support apparatus for effectively containing a fragile powder substrate in a forced flow process.